1. Field of the Invention
The present invention relates to a technique for processing an image to be printed on a recording medium such as paper with use of a recording material such as ink or toner.
2. Description of the Related Art
In recent years, companies have been making proactive efforts in environmental issues and cost reduction. In such a context, a printing industry has been required to reduce a use amount of a recording material such as ink or toner. In particular, because digital still cameras and color printers have been rapidly spread, color printing has become more convenient and closer to consumers, so that it is expected to further expand the use of such equipment. Accordingly, in the printing industry, reducing the use amount of the recording material is one of the important problems to be promptly addressed to take measures against the environmental issues and accomplish the cost reduction.
Approaches for reducing the use amount of the recording material include, for example, the following five approaches:
(1) To change a printing method from four-color printing to two-color printing to thereby reduce the number of types of recording materials. (2) To reduce a limit value of a recording material density per unit area of printing paper. (3) To reduce a density of a recording material to be used. (4) To thin pixels necessary to form an image. (5) To replace mixed color of cyan, magenta, and yellow by black.
In the color printing, colors are expressed by combinations of three color inks of cyan, magenta, and yellow, or the like. All colors can be expressed by the three colors of cyan, magenta, and yellow in theory; however, in practice, deep and bright black color cannot be expressed. For this reason, in full-color printing, the deep and bright black color is expressed by adding black ink. In the above approach (1), such four color inks of cyan, magenta, yellow, and black are reduced in number to two color inks to thereby reduce the use amount of the recording material. However, this approach cannot express many of colors perceivable by human beings, so that image quality of a printed material becomes very poor.
In the approaches (2) to (4), the density of the recording material is simply reduced to thereby suppress the use amount of the recording material. For example, pieces of pixel data are thinned to thereby reduce the number of pixels necessary to form an image, resulting in the suppression of the use amount of ink (see Japanese Patent Laid-Open No. H11-112789. However, in these approaches, the recording material density per unit area of printing paper is only equally reduced, so that clearness and contrast of a printed material are reduced.
The approach (5) is referred to as UCR (Under Color Removal). The UCR utilizes a characteristic feature that even if an under color among cyan, magenta, and yellow, which is the color having the lowest density value among the three colors, is replaced by a black color having the same density value as the lowest density value, color tone (hue) is not much different. The use of the UCR enables the consumption amount of the recording material to be reduced without deteriorating the image quality too much. On the other hand, the density of the recording material applicable onto the printing paper has a limit (limit value). Accordingly, in a printing area in which dark color should be expressed, the UCR is typically used to reduce the consumption amount of the recording material.
The use of the UCR enables the consumption amount of the recording material to be reduced without deteriorating the image quality too much; however, in recent years, an approach for reducing the consumption amount of the recording material with use of a color space compression technology along with the UCR is also considered (see Japanese Patent Laid-Open No. 2006-68982). The color space compression is a technology for compressing color into a narrow color reproduction range from a wide color reproduction range, and used when an image displayed on a device having the wide color reproduction range is output with use of a printer having the narrow color reproduction range. For example, when an image displayed on a liquid crystal display is output with the printer, the color space compression is applied. In Japanese Patent Laid-Open No. 2006-68982, by using the color space compression to convert high saturation color into low saturation and high brightness color, a black replacement ratio in the UCR is increased. This enables the consumption amount of the recording material to be reduced while suppressing the change in color tone of the image.
However, the reduction in consumption amount of the recording material only by the UCR has a limit. Also, the approach disclosed in Japanese Patent Laid-Open No. 2006-68982 described above has a problem that saturation and brightness of the image are respectively decreased and increased at the time of the color conversion, and therefore clearness of the image is spoiled and also contrast is reduced, resulting in unclearness of the printed image.